This invention relates to zero-current switching, forward power conversion.
One such power converter (e.g., the one described in Vinciarelli, U.S. Pat. No. 4,415,959, issued Nov. 15, 1983, assigned to the same assignee as this application, incorporated herein by reference) transfers energy from a voltage source for delivery to a load using a transformer that has a controlled amount of effective secondary leakage inductance (e.g., a leakage reactance transformer). On the source side of the transformer, a first switch is connected in series with the source and the primary winding of the transformer. The first switch connects the source to and disconnects it from the primary winding in a succession of energy transfer cycles. On the load side of the transformer, a first unidirectional conducting device and a capacitor are connected in series with the secondary winding. The capacitor and the effective leakage inductance define a characteristic time scale for the cycling of the first switch such that the first switch is cycled on and off at times when the current in the effective leakage inductance and the first unidirectional conducting device are essentially zero. The first unidirectional conducting device constrains current flow in the effective leakage inductance to be directed only in the direction of the load, thereby preventing bidirectional energy flow (resonance) from occurring between the effective leakage inductance and the capacitor. Energy is transferred to the load via a second inductor whose value is large in comparison with the effective leakage inductance. This second inductor effectively appears as a current sinking load across the capacitor. A second unidirectional conducting device connected in parallel with the capacitor prevents charging of the capacitor when the capacitor voltage returns to zero and current flowing in the second inductor is in the direction of the load. In the topology so described, the parameters of the circuit elements and the requirement of zero-current switching constrain the converter to unidirectionally transfer an essentially fixed amount of energy during every energy transfer cycle. Because the output power is the product of that fixed amount of energy multiplied by the frequency of occurrence of the energy transfer cycles, maintaining an essentially fixed load voltage as the converter load is varied requires that the frequency of occurrence of the energy transfer cycles be varied essentially in proportion to the converter load. As a result, the operating frequency (i.e., the rate of occurrence of energy transfer cycles) of such a converter will tend to vary widely as load is varied.
One way of reducing the operating frequency range of a zero-current switching forward converter is disclosed in Vinciarelli, "Zero-Current Switching Forward Power Conversion With Controllable Energy Transfer", U.S. patent application Ser. No. 07/799,675, filed Nov. 21, 1991, now abandoned (incorporated herein by reference). In the converter circuit disclosed therein, the second unidirectional conducting device is replaced with a bidirectional load-side switch (or a second unidirectional switch is placed in parallel with the second unidirectional conducting device). By opening and closing the bidirectional switch at appropriate times, in synchronism with the opening and closing of the first switch, the amount of forward energy transferred during a converter operating cycle may be controlled on a cycle-by-cycle basis. In one operating mode, referred to as reverse boost mode, the energy transferred during each operating cycle is reduced (relative to the energy which would be transferred in a zero-current switching converter without the bidirectional switch) as load is reduced. As a result, as load is reduced, a converter operating in the reverse boost mode must produce more energy transfer cycles than an unboosted converter in order to deliver the same amount of power to the load. Thus, the minimum operating frequency of a converter operating in reverse boost mode is higher than the minimum operating frequency of an unboosted converter, and the variation in operating frequency with load is reduced.